1. Field of the Invention
The present invention relates to a technique for forming a metal layer and, more particularly, to a method for forming a metal layer by depositing aluminum (Al)-based alloy on a wafer by sputtering, and a semiconductor device formed by using that method.
The metal layer forming technique according to the present invention can be widely used in the process of forming an electrical wiring for an integrated circuit (IC) or a large scale integration (LSI).
2. Description of the Related Art
Conventionally, an Al or in Al-based alloy such as Al-1% by weight of silicon (Si), Al-2% by weight of copper (Cu), Al-1% by weight of Si-0.5% by weight of Cu, Al-1% by weight of Si-0.1% by weight of titanium (Ti), and the like, are employed as the wiring materials for LSI's, since such alloys contribute to a decrease in the power dissipation, due to the low resistance thereof. The wiring layers using these alloys are usually formed by sputtering. Further, recent trends toward a higher integration and higher density of a semiconductor device, have led to the formation of a very steep step portion of a layer on which an Al wiring layer is formed. Therefore, considering, for example, a contact hole in the device, the ratio of the step height thereof to the width of the holes thereof is great. Accordingly, it is difficult to form an Al wiring layer having a good step coverage, i.e., a satisfactory degree of thickness, conforming to the shape of a layer thereunder.
A known technique of piling a plurality of Al wiring layers as a multiple layer stack provides a higher integration and higher density device. In this case, however, a problem arises in that, if the step coverage of an Al wiring layer is not good, an adverse influence is exerted upon the step coverage of an insulation layer to be formed thereupon and, furthermore, this adverse influence is extended to the step coverage of another Al wiring layer to be formed thereon, and thus it is difficult to obtain a multiple-layer wiring construction having a good shape.
To cope with this problem, a metal layer must be deposited to obtain a planarized shape, particularly at a steep step portion, and thus obtain a good step coverage in the step portion. As a technique therefor, a method has been proposed of carrying out a process of sputtering while the temperature of a wafer is kept high. According to this method, since the surface of the wafer is heated, Al atoms deposited on the wafer can be easily moved on the surface thereof after the deposition process is completed, and as a result, the step coverage of the layer at the step portion can be improved. However, if the temperature of the wafer is too high, another problem arises as follows.
When the temperature of the wafer is raised to a predetermined temperature and sputtering is effected, an admixture contained in the Al-based alloy, e.g., atoms of Si, Cu, and the like, precipitates in the vicinity of the interface between the Al wiring layer and the layer thereunder, e.g., a semiconductor layer, to such an extent that this precipitated admixture cannot be completely removed in a later process. Accordingly, this causes difficulties in, for example, a patterning of the Al wiring layer at the later stage, because it is technically difficult to remove both an unnecessary Al metal portion and the precipitated admixture such as Si, even if the patterning process is carried out by dry etching.
Also, as another technique of heating a wafer to form a wiring layer, a method is known in which a bias voltage of -500 to -1000 volts is applied to the wafer. According to this method, however, since the force of impact of Ar atoms incident on the wafer is very strong, Al atoms once deposited on the wafer are resputtered therefrom, and the resputtered Al atoms are deposited again. As a result, a drawback arises in that "roughness" occurs on the surface of the Al wiring layer and a white turbidity appears thereon, resulting in a deterioration in the quality of the layer.